Reproduction
627
in follicle development, induces the changes in the accessory
sex organs and secondary sex characteristics associated with
puberty.
Menarche,
the fi rst menstruation, is a late event of
puberty (averaging 12.3 years of age in the United States).
As in males, the mechanism of the brain change that
results in increased GnRH secretion in girls at puberty
remains unknown. The brain may become less sensitive to
the negative feedback effects of gonadal hormones at the
time of puberty. Also, the adipose-tissue hormone leptin
(Chapter 16) is known to stimulate the secretion of GnRH
and may play a role in puberty. This may explain why the
onset of puberty tends to correlate with the attainment of
a certain level of energy stores (fat) in the girl’s body. The
effect
of
leptin
on
GnRH
could
also
exp
lain
why
g
ir
ls
who exercise extensively and are extremely thin (decreased
leptin) often fail to initiate menstrual cycles at the expected
age (delayed menarche). The onset of puberty is not abrupt
but develops over several years, as evidenced by slowly rising
plasma concentrations of the gonadotropins and testoster-
one or estrogen.
Female Sexual Response
The female response to sexual intercourse is characterized by
marked increases in blood fl ow and muscular contraction in
many areas of the body. For example, increasing sexual excite-
ment is associated with vascular engorgement of the breasts
and erection of the nipples, resulting from contraction of
smooth muscle fi bers in them. The clitoris, which has a rich
supply of sensory nerve endings, increases in diameter and
length as a result of increased blood fl ow. During intercourse,
the blood fl ow to the vagina increases and the vaginal epithe-
lium is lubricated by mucus.
Orgasm in the female, as in the male, is accompanied
by pleasurable feelings and many physical events. There is a
sudden increase in skeletal muscle activity involving almost all
parts of the body; the heart rate and blood pressure increase,
and there is a transient rhythmical contraction of the vagina
and uterus. Orgasm seems to play a minimal role in ensuring
fertilization because conception can occur in the absence of
an orgasm.
Sexual desire in women is probably more dependent upon
androgens, secreted by the adrenal glands and ovaries, than
estrogen. Sex drive is also maintained beyond menopause, a
time when estrogen levels become very low. New studies have
suggested that low-dose androgen therapy may be useful for
the treatment of decreased libido in women. These effects are
mediated by a direct effect of androgen and by conversion of
androgens to estrogen by aromatase in the brain.
Pregnancy
For pregnancy to occur, the introduction of sperm must occur
between fi ve days before and one day after ovulation. This is
because the sperm, following their ejaculation into the vagina,
remain capable of fertilizing an egg for up to four to six days,
and the ovulated egg remains viable for only 24 to 48 h.
Egg Transport
At ovulation, the egg is extruded onto the surface of the ovary.
Recall that the fi mbriae at the ends of the fallopian tubes are
lined with ciliated epithelium. At ovulation, the smooth mus-
cle of the fi mbriae causes them to pass over the ovary while the
cilia beat in waves toward the interior of the duct. These ciliary
motions sweep the egg into the fallopian tube as it emerges
onto the ovarian surface.
Within the fallopian tube, egg movement, driven almost
entirely by fallopian-tube cilia, is so slow that the egg takes
about four days to reach the uterus. Thus, if fertilization is to
occur, it must do so in the fallopian tube because of the short
viability of the unfertilized egg.
Intercourse, Sperm Transport, and Capacitation
Ejaculation, described earlier in this chapter, results in depo-
sition of semen into the vagina during intercourse. The act
of intercourse itself provides some impetus for the transport
of sperm out of the vagina to the cervix because of the fl
uid
pressure of the ejaculate. Passage into the cervical mucus by
the swimming sperm is dependent on the estrogen-induced
changes in consistency of the mucus described earlier.
Sperm can enter the uterus within minutes of ejaculation.
Furthermore, the sperm can survive for up to a day or two
within the cervical mucus, from which they can be released to
enter the uterus. Transport of the sperm through the length of
the uterus and into the fallopian tubes occurs via the sperm’s
own propulsions and uterine contractions.
The mortality rate of sperm during the trip is huge.
One reason for this is that the vaginal environment is acidic,
a protection against yeast and bacterial infections. Another is
the length and energy requirements of the trip. Of the several
hundred million sperm deposited in the vagina in an ejacula-
tion, only about 100 to 200 reach the fallopian tube. This is
one of the major reasons there must be so many sperm in the
ejaculate for fertilization to occur.
Sperm are not able to fertilize the egg until they have
resided in the female tract for several hours and been acted
upon by secretions of the tract. This process, called
capacita-
tion,
causes: (1) the previously regular wavelike beats of the
sperm’s tail to be replaced by a more whiplike action that pro-
pels the sperm forward in strong surges, and (2) the sperm’s
plasma membrane to become altered so that it will be capable
of fusing with the surface membrane of the egg.
Fertilization
Fertilization
begins with the fusion of a sperm and egg, usu-
ally within a few hours after ovulation. The egg usually must be
fertilized within 24 to 48 hours of ovulation. Many sperm, after
moving between the cumulus of granulosa cells still surround-
ing the egg, bind to the zona pellucida. The zona pellucida
proteins function as receptors for sperm surface proteins. The
sperm head has many of these proteins and so becomes bound
simultaneously to many sperm receptors on the zona pellucida.
This binding triggers what is termed the
acrosome
reaction
in the bound sperm: the plasma membrane of the
sperm head is altered so that the underlying membrane-bound
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